This application includes an Appendix containing computer code that performs halftoning of images in accordance with this invention.
1. Field of Invention
This invention relates to techniques for printing polychromatic continuous tone images. More particularly, this invention is directed to systems and methods for halftoning images for high quality color printing using line screening for color image separation layers and hexagonal dot screening for the black image separation layer.
2. Description of Related Art
Conventional halftoning adds a two-dimensional, spatially periodic, dot screen or line screen structure to the images to be halftoned. Typically, the same screen, or at least a number of essentially identical screens, are used to halftone each of the color image separation layers of a polychromatic, i.e., color, image. However, the halftone screens are oriented at different angles for printing the respective halftone color image separation layers.
Xerographic images are ordinarily printed for reflective mode viewing. Accordingly, xerographic color images typically are composed by printing in superimposed registration a number of subtractive primary color image separation layers usually together with a neutral image separation layer that characterizes the luminescence of the source image. The subtractive primary colors are usually cyan (C), magenta (M), and yellow (Y), while the neutral is usually black (xe2x80x9cKxe2x80x9d). Additionally, other image separation layers for even further characterizing the source image can be used. Printing in superimposed registration is performed by overlaying the image separation layers produced by the halftone screens at different angles.
Digital halftoning has evolved as a method of rendering the illusion of continuous tone, or xe2x80x9ccontonexe2x80x9d, images using devices that are capable of producing only binary picture elements. However, digital halftoning can suffer from misregistration between the various color image separation layers used in color images, for example, cyan, magenta, yellow and black (CMYK). This misregistration can be caused by misalignment among the various halftone screens and also by misalignment between the halftone screens and an image forming apparatus grid structure, i.e., the output grid structure, used to generate electronic image data from an image on an image forming member. It can also include errors in rotation in the screen angle. The misregistration can cause moirxc3xa9 patterns, which are detrimental to the accurate rendering of the color image.
Moirxc3xa8 patterns are xe2x80x9cbeatingxe2x80x9d, i.e., periodially mismatching, patterns of interference that degrade resulting rendered images. When the overlaid halftone screens provide different component colors, as during rendering of a multicolor image, the moirxc3xa8 pattern can result in a color shift or variation in tone.
Substantial effort and expense have been invested in minimizing the moirxc3xa8 patterns caused by halftoning techniques for producing binary renderings of contone images. Misregistration, improper screen angle and improper screen frequency can increase the halftone screens"" susceptibility to moirxc3xa8 patterns. Additionally, because the moirxc3xa8 patterns can be caused by halftone screens beating with the output grid structure, the moirxc3xa8 pattern may be caused by a difference between the halftone screen pitch frequencies and the re-sampling rate frequency within the image forming apparatus. Even minor variations in dot position caused by systematic errors such as quantization round-off errors can produce moirxc3xa8 patterns resulting from beat frequencies between the periodic screens.
The perceived quality of the resulting color image is strongly dependent on the precision with which the color image separation layers are spatially registered with each other and the precision with which the halftone screens are oriented in relationship to a scan grid used by the image forming apparatus.
Additionally, conventional halftoning methods adjust or warp the image data produced by an image data generator, such as a grayscale image generator, or binary image generator, to minimize moirxc3xa8, such as those disclosed in U.S. Pat. No. 5,732,162 to Douglas M. Curry, incorporated herein by reference in its entirety and U.S. Pat. No. 4,537,470 to Schoppmeyer.
However, merely warping the image data to minimize the moirxc3xa8 patterns results in offsets within the image data which have no corresponding adjustment or warp in the halftone screens used to render the color image separation layers. Therefore, moirxc3xa8 pattern minimization is conventionally improved by also warping halftone screens in a halftone screen system to correspond to the warping of the image data, as disclosed in U.S. Pat. No. 5,485,289 to Curry, incorporated herein by reference in its entirety. The ""289 patent provides a detailed discussion of warping both image data and halftone screens.
Another conventional method for minimizing moirxc3xa8 maximizes the screen angles between the halftone screens. This is done because increasing the screen angles reduces the prominence of moirxc3xa8 because interference between the image separation layers is more frequent but to a lesser degree.
Initially, maximizing the angular displacement between the screens to minimize moirxc3xa9 might suggest that line screens should be used for halftoning color images, because a line screen is rotationally symmetric only every 180xc2x0, while a dot screen is rotationally symmetric every 90xc2x0.
Thus, for example, when printing a four color (CMYK) halftoned image with a line screen, an average allowable separation-to-separation screen displacement angle is 45xc2x0. This means that the respective image separation layers may be printed with a line screen oriented at, for example, 0xc2x0, 45xc2x0, 90xc2x0 and 135xc2x0 relative to each other.
Moirxc3xa9 can degrade the image when the color image separation layers that are screened at relative orientations of 0xc2x0 and/or 90xc2x0 overlap the color image separation layers that are screened at relative orientations of 45xc2x0 and/or 135xc2x0. Alternatively, if a dot screen is used for halftoning the four color image separation layers of the image, the average allowable screen displacement angle is 22.5xc2x0. As a result, there is more opportunity for moirxc3xa9 when the screen orientations are closely spaced.
Just as errors in frequency and angle can cause moirxc3xa9, so can imperfections in the scanner, or set of scanners, which provide the scan structure for the printer. If the respective scan structure for all four color layers do not exactly overlap, the halftones can be mis-registered so as to be another source of moirxc3xa9.
The halftone marks, especially dots, must periodically be placed so that the centers of the dots do not exactly line up with the scan structure of the marking device to make halftones at exactly the right frequency or angle or to compensate for non-overlapping scan structures to avoid the pitfalls of moirxc3xa9. This offset of the halftone marks from the scan structure is referred to as being out of phase. Obviously, not all frequencies, angles or registration corrections may be printed by the marking device without periodically being forced to place marks out of phase.
Increasing the quantization of the marking device can provide more in-phase places to place marks. Typically, a laser marking device, such as a polygon scanner, favors the fast scan direction with high quantization over the process or scan pitch direction, which generally has a coarser quantization. Therefore, halftone dots, which go out of phase in two dimensions when rotated at some arbitrary angle or warped to compensate for some arbitrary error, are more difficult to place in their proper positions because the process direction limits their placement precision. A line screen, on the other hand, can be made to be more or less perpendicular to the fast scan direction, which is the highest quantization direction, so that the line screen mark can be placed with higher precision in that direction.
Despite the greater average screen displacement angle provided by using line screens, dot screens generally are preferred for printing color halftone images because dot screen structures usually provide a greater dynamic tone range. Thus, this invention provides tone-variation-resistant phase-shiftable halftone screen systems and methods that perform color image halftoning.
The invention separately provides systems and methods that adjust the image data in conjunction with adjusting the halftone screens.
This invention separately provides systems and methods that adjust the image data and the halftone screens based on imperfections of an image forming apparatus to provide improved image rendering.
The invention separately provides systems and methods that halftone multicolor images by rendering the hexagonal dot screen image separation layer without warping the hexagonal halftone screen, while warping two of the line screens to register the resulting line screened image separation layers with the dot screened image separation layer.
The tone-variation-resistant halftone screen systems and methods of this invention use the relatively large average screen displacement angles offered by line screening while providing a dynamic tone range performance similar to that of the performance of dot screening. In other words, the halftone screen systems and methods of this invention combine the best features of line screening and dot screening for rendering halftone color images.
An exemplary embodiment of the methods and systems according to the invention uses line screens for halftoning the chromatic image separation layers of color images and a hexagonal dot screen for halftoning the luminescent image separation layer. For a four separation layer color image, two of the three line screens are tilted, i.e., oriented at other than 90xc2x0 angles, from the horizontal or vertical axes. This orientation allows for easy adjustment of vertical positions of rendered marks by horizontally adjusting the horizontal position of the tilted lines screens. Therefore, dot position in the vertical, i.e., slow-scan, direction, can be adjusted by moving the tilted line screen in the horizontal, i.e., fast-scan, direction. This improves operation because adjustment of the line screens in the fast-scan direction is easier to control.
For example, when printing CMYK halftone images, a hexagonal dot screen is used to print the luminescent image separation layer, e.g., the black image separation layer. The black image separation layer is registered with and printed to be coincident with the output grid structure by registering the hexagonal dot screen with the output grid structure. The chrominance image separation layers, e.g., cyan, magenta and yellow, are each printed using line screens. The two tilted line screens carry the cyan and magenta layers, and are one-dimensionally warped to be concentric or collateral with the periodic hexagonal lattice structure resulting from registering the hexagonal dot screen with the output grid structure. The third line screen, which is parallel with the scanning laser, is also one dimensionally warped to coincide with the hexagonal dot lattice, but since it is carrying the yellow layer, it is less susceptible to the effects of moirxc3xa9 than the cyan and magenta layers. Therefore, the three line screens become concentric with the hexagons defined by the hexagonal lattice structure to maximize the dynamic tone range and minimize moirxc3xa9 of the luminescent image separation layers.
The adjustment of the halftone screens provided by the systems and methods of this invention is an improvement over conventional methods for warping halftone screens because the adjustment provided by the systems and methods of this invention involves only one-dimensional warping of the tilted line screens, rather than the two-dimensional warping of halftone dot screens as previously required in, for example, the methods disclosed in the incorporated ""289 patent.
The systems and method of this invention may be advantageously used in conjunction with electronic registration of the image separation layers using tagged, antialiased bytemaps provide improved image rendering. However, using such tagged, antialiased bytemaps is not necessary.
These and other features and advantages of this invention are described in, or are apparent from, the following description of the apparatuses and methods according to this invention.